I have a website about places and attractions in my country. Each place/attraction is a different custom post. On this page, i would like to add a button with the value "Check In" and if clicked by a registered user i would like to save a value in db with the: user id, post id and 1 if the button was pressed.

On the user page, i would like to show where he check in and how many times.
I have allready the user page, the places pages, etc. I only need to add this feature.

It's doable? If yes, can someone help me?

LE: I don't expect that someone will just give me the code. Just point me in right direction - how to add for a custom post a custom entry in the db.

Yes, i've read all the google :P - i've searched and reseached but i can find anything concrete. The ideea it's simple - when a registered user clicks a button, i need to save his name and the custom post id in the db. But i've run into a block when i've tried to save anything in db.
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Mircea MoldovanApr 18 '13 at 17:10